Nomenclature: IUPAC rules for nomenclature.

The idea

Naming is a fixed algorithm. Find the longest continuous carbon chain that contains the principal functional group and name it from the stem: meth, eth, prop, but, pent, hex. Add the suffix for that group, such as -ane, -ene, -ol, -al, -one or -oic acid. Number the chain from whichever end gives the lower locant to the functional group first and then to the substituents. List substituents as prefixes in alphabetical order, using di, tri and tetra for repeats but ignoring those multipliers when alphabetising, so ethyl comes before dimethyl. Punctuate with hyphens between numbers and letters and commas between numbers, as in 2,3-dimethylpentane. In naming questions AQA will not accept a phonetic misspelling.

What earns the marks

• The correct stem taken from the longest chain containing the functional group.
• The correct suffix and locant for the principal functional group.
• Substituent prefixes in alphabetical order with correct locants.
• Correct punctuation, with hyphens between numbers and letters and commas between numbers.

Common mistakes and how to put them right

• Numbering from the wrong end, so writing pent-4-ene rather than pent-1-ene.

• Missing a longer chain that turns a corner in a skeletal structure.

• Alphabetising by the multiplier, so placing dimethyl under d instead of methyl under m.

• Writing 2 methylpentane or 2,3 dimethylpentane without the hyphens the convention requires.

Exam tips

• Count every possible chain before choosing the longest, because branched drawings hide it.
• Write the name out fully and spell it correctly, since naming questions do not accept phonetic spelling.
• Check the locant set is the lowest possible by numbering from both ends.